The present invention relates to a novel photocurable organopolysiloxane composition which is suitable, for example, for providing cured surface films with excellent releasability on various substrate materials by coating followed by irradiation with light rich in ultraviolet.
In particular, the invention relates to a novel photocurable organopolysiloxane composition utilizing the photo-induced crosslinking reaction between a first organopolysiloxane having at least two mercaptoalkyl groups directly bonded to the silicon atoms in a molecule and a second organopolysiloxane having at least two alkenyl groups directly bonded to the silicon atoms in a molecule sensitized with a photosensitizer of a novel type.
The above described type of photocurable organopolysiloxane compositions comprising a mercaptoalkyl-containing organopolysiloxane and an alkenyl-containing organopolysiloxane admixed with a photosensitizer is well known in the art (see, for example, Japanese Patent Disclosure No. 50-61386). Some of the prior art photosensitizers suitable for use in the above described type of the photocurable compositions are exemplified by acetophenone, benzophenone, 4-chlorobenzophenone, 4,4'-dimethoxybenzophenone, 4-methylacetophenone, azobisisobutyronitrile and benzoin ethyl ether (see, for example, U.S. Pat. No. 4,052,529).
On the other hand, it is known that photopolymerization of certain kinds of photopolymerizable organic compounds having carbon-carbon double bonds can be sensitized by several trialkylsilylesters of .alpha.-substituted or unsubstituted benzoin esters represented by the general formula ##STR2## wherein R represents hydrogen, lower alkyl or alkenyl having 1 to about 4 carbon atoms, R' represents lower alkyl having 1 to about 4 carbon atoms, preferably methyl, and R" and R"', which may be identical or different, each represent hydrogen, halogen, lower alkyl having 1 to about 4 carbon atoms or methoxy group, as exemplified by benzoin-trimethylsilyl ester, .alpha.-methylbenzoin-trimethylsilyl ester and the like (see, for example, U.S. Pat. No. 3,636,026).
One of the problems in the above described photosensitizers, when used in the photocurable compositions comprising a mercaptoalkyl-containing and an alkenyl-containing organopolysiloxanes, is the poor compatibility of them with the organopolysiloxanes bringing about several disadvantages that relatively prolonged irradiation with light is necessitated to obtain complete curing and, moreover, evenness or uniformity in the photocuring can hardly be expected throughout whole areas under irradiation, that the eventual precipitation of the photosensitizer in the coating films of the cured composition results in the decreased adhesion of the films to the substrate surface as well as in an inferior releasability of the cured films to sticky substances when the cured films are formed with an object of imparting releasability, and that the presence of an organic solvent, which is necessarily formulated in the composition to accelerate the dispersion of the photo-sensitizer with low compatibility, is detrimental to the application of the composition to substrates liable to dissolution in or swelling with the organic solvent. The above defects in the photocurable compositions with a sensitizer can only be partly overcome even with the benzoin-silyl ester compounds.